• Michael B Rothberg, Peter B Imrey, Ning Guo, Abhishek Deshpande, Thomas L Higgins, and Peter K Lindenauer.
• Center for Value-Based Care Research, Community Care, Cleveland Clinic, Cleveland, Ohio, USA.
• J Hosp Med. 2022 Aug 1; 17 (8): 624-632.

BackgroundGuidelines recommend testing hospitalized patients with community-acquired pneumonia (CAP) for Legionella pneumophila only if the infection is severe or risk factors are present. There are no validated models for predicting Legionella.ObjectiveTo derive and externally validate a model to predict a positive Legionella test.Design, Setting And ParticipantsDiagnostic study of adult inpatients with pneumonia using data from 177 US hospitals in the Premier Healthcare Database (training and hold-out validation sets) and 12 Cleveland Clinic Health System (CCHS) hospitals (external validation set). We used multiple logistic regression to predict positive Legionella tests in the training set, and evaluated performance in both validation sets.Main Outcome And MeasuresThe outcome was a positive Legionella test. Potential predictors included demographics and co-morbidities, disease severity indicators, season, region, and presence of a local outbreak.ResultsOf 166,689 patients hospitalized for pneumonia, 43,070 were tested for Legionella and 642 (1.5%) tested positive. The strongest predictors of a positive test were a local outbreak (odds ratio [OR], 3.4), June-October occurrence (OR, 3.4), hyponatremia (OR, 3.3), smoking (OR, 2.4), and diarrhea (OR, 2.0); prior admission within 6 months (OR, 0.27) and chronic pulmonary disease (OR, 0.49) were associated with a negative test. Model c-statistics were 0.79 in the Premier and 0.77 in the CCHS validation samples. High-risk patients were only slightly more likely to have been tested than lower-risk patients. Compared to actual practice, the model-based testing strategy detected twice as many cases.ConclusionsAlthough Legionella is an uncommon cause of pneumonia, patient characteristics can identify individuals at high risk, allowing for more efficient testing.© 2022 The Authors. Journal of Hospital Medicine published by Wiley Periodicals LLC on behalf of Society of Hospital Medicine.

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